Visual light communication systems are known, in which LEDs are used to transmit data. Fast variations of the LED current are provided, which give rise to a varying light output, which conveys a data stream. The light fluctuations may be sufficiently rapid to avoid any visual light flicker. In this way, lighting components may communicate with each other or with a central system controller, for example to transmit sensor data wirelessly from a luminaire to a central controller.
There is a need for a suitable circuit that can be used to modulate the LED current (or the current of any other current-drive lighting load) with data. However, a standard off-the-shelf driver delivers a constant AC current. This makes it difficult to modulate the current through the LED in order to provide the desired modulated light output.
FIG. 1 shows in schematic form three possible general approaches for providing a modulated output current to an LED load 10.
A first approach is to achieve current modulation based on modified control of the main driver, which is typically a switch mode power converter. An analogue input voltage “Vref” or else a digital pulse width modulation disable signal “PWM disable” controls a transmission gate 12 at the output of the driver. This works well for slow data, for example data at less than 20 kbits/second, corresponding to the switching frequency of the switch mode power converter.
A second approach is to use a parallel shunt 14. This turns off the LED by providing a bypass for the LED current. This can waste a substantial amount of energy.
A third approach is to use a linear modulator 16. This provides a current modification by controlling a transistor which functions as a current regulator in series with the LED load 10. This is a more power-efficient solution.
The output of the driver has an output capacitor. For the first and second approaches above, the output capacitor should be very small, whereas for the third approach, it should be very large.
Implementation of a linear modulator is possible for a driver which delivers an output voltage. In this case, the linear modulator functions as a current control element for controlling the current between fixed voltage rails. For example, it comprises a MOS transistor with a controlled source-drain current. The resulting current flowing may be used as a feedback control parameter to control the modulation function.
However, standard LED drivers deliver a regulated current rather than voltage. There is then incompatibility between the main driver regulated output current and the local current control.
In particular, a current-regulating LED driver in series with a current-modulating transistor is known to be difficult to control. Basically, the circuit comprises two current sources in series: the LED driver which is the source and the current modulator that acts as the load. With a minor mismatch between these two currents, clipping of the modulation current may occur or a rapid escalation of output voltage of the driver may occur that also causes dramatic power losses in the current modulator.
EP 2 547 174 shows the use of a linear modulator according to the third approach wherein a current feedback loop is provided to the driver for maintaining a constant average current in the LEDs. The feedback loop of EP 2 547 174 comprises a low pass filter in such a way that the modulated current is minimized in the feedback loop. Such feedback loop does not compensate a possible drift in the output voltage of the driver because it only takes into account the average current.
Other modulation possibilities are shown in US 2015/0115809 but none of them indicates how to compensate the voltage drift at the output of a standard current driver when a modulating element is serially mounted with the LEDs.
There is therefore a need for a circuit design which enables the efficiency benefits of a linear modulator to be obtained, but which also allows a standard and hence low cost regulated current driver to be used.